The neurohypophyseal hormones vasopressin and oxytocin play pivotal roles in blood pressure regulation, water retention by the kidneys, and in childbirth. These two hormones are related nonapeptides that contain tyrosine in the second position. Although considerable information about hormone structure and activity is known, and the X-ray structure of an oxytocin analog has been determined, little is understood about the control of hormone synthesis and storage as well as how these peptide hormones interact with cell surface receptors and invoke the biochemical response. Considerable insight on the binding reaction of the hormone with its receptor will be gained from these proposed studies. This proposal outlines the approaches that will be used to study the binding interactions. Fluorescence spectroscopic techniques will be used since these methods provide the sensitivity, experimental parameters, and the ability to work in aqueous solution that are required to study the interaction of the hormone with an isolated receptor system. The intrinsic fluorescence of the tyrosine residue in the neurohypophyseal hormones will be used to help define their structure in solution. This will involve examining both steady- state and time-dependent fluorescence parameters of the hormones and synthetic analogs for comparison with existing structural information. Hormone analogs, derivatized with another fluorescent molecule of different spectroscopic properties, will be synthesized and characterized by fluorescence. The hormones and their analogs will then be used in binding studies with neurophysin. The neurophysins are the natural "carrier" for neurohypophyseal hormone transport after synthesis to storage in the pituitary. The neurophysins also have only a single tyrosine and no tryptophan, enabling the characterization of the carrier proteins by fluorescence as well as the ability to monitor structural changes in both the hormone and the protein during the binding interaction. Thus, neurophysin will be a model receptor, enabling the development of the methodologies to study neurohypophyseal hormone binding interactions with real receptors when they are isolated.